1. Field of the Invention
This invention relates to a charging roller, a process cartridge and an electrophotographic apparatus. More particularly, it relates to a charging roller used in the contact charging of an electrophotographic photosensitive member; and a process cartridge and an electrophotographic apparatus which have such a charging roller.
2. Related Background Art
In recent years, as charging assemblies for electrophotographic photosensitive members, contact charging assemblies have been put into practical use. Compared with corona charging conventionally used, contact charging can control the generation of ozone involved with the charging of electrophotographic photosensitive members and can reduce power consumption necessary for the charging. Then, in particular, a roller charging system making use of a charging roller as a charging member is preferably used in view of the stability of charging.
In the roller charging system, a conductive elastic roller is brought into pressure contact with a member to be charged and a voltage is applied thereto to charge the member to be charged.
The charging is performed by causing electric discharge from a charging member (charging roller) to the member to be charged, and hence the charging of the member to be charged takes place upon application of a voltage not lower than a certain threshold voltage. For example, when the charging roller is brought into pressure contact with an organic photosensitive member (OPC) having a 25 μm thick photosensitive layer, the surface potential of the photosensitive member begins to rise upon application of a voltage of about 640 V or above as absolute value and, at voltages above that voltage, the photosensitive member surface potential linearly increases at an inclination 1 with respect to the applied voltage. This threshold voltage is hereinafter defined as charging start voltage Vth.
That is, in order to attain a photosensitive member's surface potential Vd considered necessary for electrophotography, a DC voltage of Vd+Vth higher than a voltage necessary for image formation must be applied to the charging roller. Such a method of applying only DC voltage to the contact charging member in this way is called DC charging.
In the DC charging, however, any environmental variations may cause variations in electrical resistance of the charging member, and also gradual changes in photosensitive-layer thickness caused by scrape of the photosensitive layer may cause variations in the Vth. For these reasons, it has not been easy to control the potential of the electrophotographic photosensitive member to the desired value,
Accordingly, in order to achieve more uniform charging, an AC+DC charging system is used in which a voltage formed by superimposing an AC component with a 2×Vth or higher peak-to-peak voltage on a DC component corresponding to the desired Vd e.g., Japanese Patent Application Laid-Open No. S63-149669. This is a system aiming at a potential-leveling effect which is attributable to AC, where the potential of the member to be charged converges to the Vd, the middle of the peak of AC voltage, and may hardly be affected by external disturbance such as environmental variations.
As the charging roller, there is an example in which using a conductive seamless tube a surface layer is formed on a conductive support member e.g., U.S. Pat. No. 4,967,231. There are also disclosed a seamless composed of a fluorine resin e.g., Japanese Patent Application Laid-Open No. H05-2313, and a multi-layer tube formed of layers having different conductivities e.g., Japanese Patent Application Laid-Open No. H05-96648. As a method for manufacturing such charging rollers, a method for formation by insertion is mentioned as the above conventional techniques. Also there is disclosed a method of forming the surface layer by means of a crosshead extruder e.g., Japanese Patent Application Laid-Open No. H06-58325.
Such methods of forming the charging roller by using the seamless tube enables smooth surfaces to be formed even when foam is used as an elastic layer formed on a substrate, because they may further be covered with the seamless tube. Thus, the member to be charged can more uniformly be charged.
As a specific method by which the support member is covered with the seamless tube, a method is available in which the tube is externally fitted to the support member. Stated specifically, there is a method in which the seamless tube is formed to have an inner diameter larger than the outer diameter of the support member to be covered and then the tube is contracted by a physical or chemical means, e.g., by heat, or a method in which the seamless tube is formed to have an inner diameter smaller than the outer diameter of the support member to be covered and the tube is spread out and fitted to the support member by a physical or chemical means, e.g., by air pressure e.g., Japanese Patent Application Laid-Open No. H10-228156.
Where a developer (toner and external additives) adheres to the charging roller surface via an electrophotographic photosensitive member, the electrical resistance of the charging roller surface may change or the electrical resistance may come non-uniform, and this consequently may make it unable to charge the electrophotographic photosensitive member stably and in a good state, and furthermore to obtain good electrophotographic images. Accordingly, in order to obtain high-grade electrophotographic images stably, it is important for the charging roller to be endowed with such surface properties that can keep the developer from adhering to the charging roller surface. Here, in the charging roller having the construction in which the seamless tube is externally fitted to the periphery of an elastic layer, the present inventors have not found any examples in which the elastic layer is surely covered with the seamless tube and simultaneously more improved in surface properties.